This invention relates to an apparatus and a method for analyzing properties of a solution or solid using Raman spectroscopy and in particular to the application of Raman peak intensity ratios for analyzing and predicting the properties of a solution or solid.
In many industrial chemical processes, the amount of reactants, or input components, that are used is less than or more than the amount necessary to carry the reaction to the point of obtaining a desired characteristic(s) of the product stream. If too little of the input component is used, often the desired target value of a characteristic from the process is not obtained. Alternatively, if an excessive amount of an input component is used, the desired characteristic may be obtained, but the excess input component is typically released as waste in the effluent of the process. In other cases, excessive amounts of an input component may cause undesirable reactions to occur that produce unwanted characteristics. Further, the wasted input component is economically costly and can become an environmental pollutant if it is released into the environment without being removed or recycled from the effluent. The difficulty in controlling chemical processes, such as bleaching, in the pulp and paper manufacturing industry can be caused by a number of factors including qualitative and quantitative variability of the pulp or wood furnish, the composition of the process chemicals, and the consistency (% wood or pulp) of the furnish. Further, changing market requirements for paper products may require a paper manufacturing operation to produce a wide variety of paper grades. New paper processing methods, equipment, and chemicals force the paper bleaching operation to adapt to these technical changes while still monitoring various characteristics of the pulp.
It is therefore desirable to be able to precisely control the input components to obtain the desired target characteristic(s) with little waste. To obtain this control, a characteristic of the effluent of an industrial process should be precisely monitored in real time in order to provide feedback control on the amount of input components, which should be added to the reactor to avoid under use or excessive use, and waste, of the input component.
For example, in the pulp and paper industry, hydrogen peroxide and the hydroperoxy anion (HO2xe2x88x92) are important input components for the oxidation and bleaching of wood pulps. In a typical pulp bleaching plant situation, the control of the bleaching chemicals is based on the brightness of the incoming pulp, the pulp flow, and the target brightness and pulp physical properties that are to be achieved. The factors of incoming pulp brightness, pulp flow, and target brightness are then used to calculate the amount of bleaching chemicals required to be added to the pulp to achieve a certain final target brightness. In another system, the brightness of the pulp is measured after bleaching chemicals are added and after allowing the reaction to occur for a defined reaction time. The resultant brightness value of the reaction is then measured and is used for feedback regulation of the bleaching chemicals.
Typically with these feedback systems, the amount of hydrogen peroxide that is used exceeds or overshoots the amount necessary to reach a final target characteristic, such as pulp final target brightness, yellowness, residual peroxide, brightness efficiency, yellowness efficiency, and delignification efficiency. The resultant unwanted variation in these pulp characteristics may cause additional processing problems in the pulp and paper processing mill. Further, in the case of peroxide bleaching, excessive use of hydrogen peroxide results in waste hydrogen peroxide in the pulp effluent, which is both costly and environmentally harmful.
In order to solve these problems the prior art has offered various solutions. For example, U.S. Pat. No. 4,878,998 teaches a method for bleaching of mechanical, thermomechanical and chemi-mechanical pulps whereby peroxide bleaching is controlled by the addition of a preset amount of bleaching chemicals at a first bleaching stage, measuring the brightness of the pulp, feed forwardly adjusting the amount of bleaching chemicals to be added at a second bleaching stage as a function of the measured brightness of the pulp from the first stage, and then bleaching the pulp at the second stage.
Canadian Patent No. 2,081,907 teaches a method and apparatus for determining information characteristics of the concentration of each of at least three intermixed components in kraft liquors having the steps of: identifying detectable characteristics that are detectable in relation to the concentration of the components, developing a mathematical relationship between the component and the characteristics, such as regression analysis, analysing a sample of solution with a UV detector, and then controlling the concentration of each of the three components by using the information from the analysis of the sample.
While current brightness sensors are able to provide a measure of the pulp brightness, they are unable to measure the bleaching efficiency of the bleaching reaction itself. Bleaching efficiency is a change in brightness of a pulp divided by the amount of peroxide consumed during the bleaching reactions. Further, measurement of yellowness efficiency, which is defined as a change in pulp yellowness divided by the peroxide consumed during the bleaching reactions, also requires a method by which the residual peroxide in the pulp effluent can be measured. Other efficiency measures relating the relative improvement in pulp optical and strength properties to the consumption of chemicals and the generation of dissolved species derived from wood are of interest, but are not readily available.
U.S. Pat. No. 5,842,150 and WO 96/122183 to Renberg et al. disclose a method, based on UV/VIS/NIR/IR, for the qualitative and quantitative determination of quality parameters in pulp and paper and/or the organic content in effluents from pulp and paper production by applying chemometric methods. Renberg et al. provide a review and discussion of the xe2x80x9cstate of the artxe2x80x9d and the need for on-line measurement of variables related to pulp and effluent quality. A discussion is provided for the use of spectral analysis with standard chemometric procedures to derive calibrated values for pulp and paper strength and brightness parameters and also measures of amounts of organic substances, for example the Total Organic Carbon (TOC), the Chemical Oxygen Demand (COD), and the Biological Oxygen Demand (BOD). The abstract and the disclosure indicate that the invention relates to UV/NIS/NIR spectroscopy, including Raman spectroscopy. However, the disclosure does not provide any examples or further disclosure with respect to the use of Raman spectroscopy. The examples disclosed in column 7 in U.S. Pat. No. 5,842,150 are all examples related to UV absorption techniques between 200 and 360 nm to some other property. It is noted that U.S. Pat. No. 5,842,150 does not disclose any wavelength region outside the UV region. Furthermore, Raman spectroscopy is an emission technique and does not extend to absorption, transmittance or reflectance techniques as discussed in U.S. Pat. No. 5,842,150. The reflectance technique disclosed therein is not the same as an emission by inelastic scattering as it occurs in Raman spectroscopy. The prior art does not relate the spectral parameters to organic indicators and does not discuss the properties related to the oxidative capacity of inorganic components that may exist in multiple oxidation states, the development of substances that contribute to scale deposition of effluent components, the physical properties of polymerizable species, such as the number of endgroups, the extent of network formation, and the chain length, or the development of bulk, yield or fiber flexibility.
The following two references, including references within, provide a general review of the application and interpretation of Raman spectroscopy with respect to lignocellulosics, Chapter 9 xe2x80x9cAn Overview of Raman Spectroscopy as Applied to Lignocellulosic Materialsxe2x80x9d by Umesh P. Agarwal from a book entitled xe2x80x9cAdvances in Lignocellulosics Characterizationxe2x80x9d, edited by Dimitris S. Argyropoulos, published by Tappi Press 1999 (ISBN 0-89852-357-5), and an article by Urnesh P. Agarwal and Sally A. Ralph in Applied Spectroscopy Volume 51, Number 11, 1997. pp 1648-1655, entitled xe2x80x9cFT Raman Spectroscopy of Wood: Identifying Contributions of Lignin and Carbohydrate Polymers in the Spectrum of Black Spruce (Picea Mariana).
Unfortunately, it is well known that there is a present lack of an appropriate method or device for the monitoring and control of pulp bleaching reaction characteristics, including pulp final target brightness, yellowness, residual peroxide, and efficiency, with respect to brightness or physical strength development, the amount of peroxide consumed, or the loss of lignin or carbohydrate substances. Also, it is known that pH measurement probes and electrochemical methods of measuring hydrogen peroxide, such as the Kajaani Polarox sensor made by Valmet Automation, can be unreliable under pH and chemical concentration conditions which are typically used for pulp brightening reactions. BTG Spectris (Sweden) has an instrument and method of measuring the peroxide concentration that employs the use of a catalyst to decompose the hydrogen peroxide to generate oxygen gas that increases the reaction vessel pressure. This instrument, the RPA-5000, then relates the change in the pressure of the reaction vessel to the concentration of peroxide. This method, while providing a badly needed measure of the peroxide concentration, is complicated and indirect and subject to variability related to sample preparation and instrument maintenance.
Measurement of the concentrations of other pulp bleaching chemicals such as sodium hydrosulfite (dithionite), chlorine dioxide, and hypochlorite present similar difficulties to those encountered to for hydrogen peroxide. In general these bleaching compounds are oxidative or reductive substances, normally existing as one or more species of inorganic oxianions in solution. A review of the current state of the art of pulp bleaching practices, including methods for measurement and control, has been published (Pulp Bleaching. Principles and Practice. Carlton W. Dence, and Duglas W. Reeve Editors. Tappi Press, Atlanta 1996).
In accordance with the invention there is provided a method for measuring an amount of peroxide or peroxyl ion of a sample comprising the following steps: (a) irradiating at least a portion of the sample with a laser light for generating a Raman spectrum of the sample; (b) obtain a Raman spectrum for obtaining at least two measurements at two different wavenumbers, a first measurement related to a Raman intensity related to an amount of peroxide or an amount of peroxyl ion, the second measurement related to the other of the amount of hydrogen peroxide and the amount of peroxyl ion; and (c) formulating a relationship between a Raman intensity for hydrogen peroxide and a Raman intensity for the peroxyl ion by comparing information related to the two measurements for determing the amount of peroxide or peroxyl ion.
In accordance with another embodiment of the invention there is provided a method for determining a property of a sample comprising the steps of: (a) irradiating at least a portion of the sample with a laser light for generating a Raman emitted light from the sample; (b) obtaining at least two measurements of the Raman emitted light between 200 cmxe2x88x921 and 4000 cmxe2x88x921, a first measurement at a first wavenumber and a second measurement at a second wavenumber; and (c) determining a non-linear relationship between the at least two measurements and the property of the sample.
Furthermore, in accordance with yet another embodiment of the present invention there is provided a method for determining a potential of an oxidative reductive process comprising the following steps: (a) irradiating at least a portion of the sample with a laser light for generating a Raman emitted light from the sample; (b) obtaining at least two measurements of the Raman emitted light between 200 cmxe2x88x921 and 4000 cmxe2x88x921, a first measurement at a first wavenumber, and a second measurement at a second wavenumber; and (c) determining a relationship between the two measurements and the potential of the oxidative reductive process. The term peak refers herein after to a maximum intensity value or a region about the maximum intensity, near or about the peak.
In accordance with the invention there is further provided a method for measuring an amount of at least one of hydrogen peroxide and peroxyl ion (HOOxe2x88x92) in a solution, comprising the steps of: irradiating at least a portion of the solution with light of a suitable wavelength and intensity to obtain information relating to a Raman spectrum thereof, said information containing data related to at least one of an intensity peak corresponding to peroxide and an intensity peak corresponding to peroxyl ion; and, processing the information to determine indicia of a concentration of at least one of hydrogen peroxide and peroxyl ion, the processing including an analysis of at least one of data related to the intensity peak corresponding to peroxide, data related to the intensity peak corresponding to peroxyl ion, a sum of data related to the intensity peaks of the peroxide and peroxyl ion, a product of data related to the intensity peaks of the peroxide and peroxyl ion, and a ratio of data related to the intensity peaks of the peroxide and peroxyl ion.
In accordance with a further embodiment of the present invention there is provided an apparatus for determining a property of a sample comprising: a laser light source for irradiating at least a portion of the sample for generating a Raman emitted light from the sample; a detector for detecting the Raman emitted light from the sample, said detector for obtaining at least two measurements of the Raman emitted light, a first measurement at a first wavenumber and a second measurement at a second wavenumber; and a processor for receiving and processing data from the detector for determining a non-linear relationship between the at least two measurements and the property of the sample.
In accordance with another aspect of the invention there is provided a system for determining a property of a sample comprising: means for determining a non-linear relationship between at least two measurements and the property of the sample, the at least two measurements corresponding to Raman emitted light between 200 cmxe2x88x921 and 4000 cmxe2x88x921, and the at least two measurements comprising a first measurement at a first wavenumber and a second measurement at a second wavenumber.
Furthermore, in accordance with the invention there is provided a system for determining a property of a sample comprising: means for comparing at least two measurements including a first measurement at a first wavenumber and a second measurement at a second wavenumber, the at least two measurements corresponding to Raman emitted light between 200 cmxe2x88x921 and 4000 cmxe2x88x921 when the sample is irradiated with a laser; means for determining a non-linear relationship between the at least two measurements and the property of the sample; and, means for determining the property of the sample in dependence upon the non-linear relationship.
In accordance with a further aspect of the invention there is provided a system for determining an amount of at least one of hydrogen peroxide and HOOxe2x88x92 in a solution, comprising: means for receiving information containing data related to at least one of a Raman intensity peak corresponding to peroxide and a Raman intensity peak corresponding to peroxyl ion; and, means for processing the information to determine indicia of a concentration of at least one of peroxide and peroxyl ion, the processing including an analysis of at least one of data related to the intensity peak corresponding to peroxide, data related the intensity peak corresponding to peroxyl ion, a sum of data related to the intensity peaks of the peroxide and peroxyl ion, a product of data related to the intensity peaks of the peroxide and peroxyl ion, and a ratio of data related to the intensity peaks of the peroxide and peroxyl ion.